1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus.
2. Description of the Related Art
In recent years, the demand for increasing capacity of a HDD (Hard Disk Drive) has been growing. In order to satisfy this demand, there is proposed a thermally assisted magnetic recording method that performs recording by heating a magnetic recording medium with a magnetic head having a laser source mounted thereon.
With the thermally assisted magnetic recording method, coercivity (coercive force) of a magnetic recording medium is significantly decreased by heating the magnetic recording medium. Therefore, a material having high magnetic anisotropy constant Ku can be used for a magnetic layer of the magnetic recording medium. Thus, size reduction of magnetic particles of the magnetic recording medium can be achieved while maintaining thermal stability. Accordingly, an area density in a class of 1 Tbit/inch2 can be attained. As an example of the material having high magnetic anisotropy constant Ku, there is proposed an ordered alloy such as an L10 type FePt alloy, an L10 type CoPt alloy, and an L11 type CoPt alloy.
Furthermore, in order to isolate crystal particles of the aforementioned alloys, an oxide (e.g., SiO2, TiO2), carbon (C), or Boron Nitride (BN) may be added to the magnetic layer as a grain boundary phase material. Owing to a granular structure having magnetic crystal particles separated at a grain boundary phase, exchange coupling between magnetic particles can be reduced. Thereby, a medium having high SN ratio can be obtained.
A non-patent document (J. Appl. Phys. 104, 023904, 2008) discloses that a magnetic particle diameter can be reduced to 5 nm by adding 38% of SiO2 to FePt. Further, the non-patent document also discloses that a magnetic particle diameter can be reduced to 2.9 nm by increasing the addition amount of SiO2 to 50%.
In order to fabricate a thermally assisted magnetic recording medium having high magnetic anisotropy, it is preferable for an L10 type ordered alloy inside the magnetic layer to have a satisfactory (001) orientation. The orientation of the magnetic layer can be controlled in accordance with an underlayer. Therefore, in order to obtain the (001) orientation, an appropriate underlayer needs to be used.
For example, Japanese Laid-Open Patent Publication No. 11-353648 discloses a L10 type FePt magnetic layer exhibiting a satisfactory (001) orientation by using a MgO underlayer.
Further, Japanese Laid-Open Patent Publication No. 2009-158054 discloses a L10 type FePt magnetic layer exhibiting a further satisfactory (001) orientation by using a MgO underlayer (serving as both a crystal orientation control layer and a thermal conductive intermediate layer) formed on a crystal particle diameter control layer having a BCC (Body Centered Cubic) structure such as a Cr—Ti—B alloy.
Further, a practical example 2.3 of Japanese Laid-Open Patent Publication No. 2012-48792 discloses an example using W-5 at % Mo/Cr as an underlayer.
Alternatively, a microwave assisted magnetic recording method is drawing attention as a next generation recording method. The microwave assisted magnetic recording method records magnetic data by radiating a microwave onto a magnetic layer of a magnetic recording medium, tilting an axis of easy magnetization, and locally reversing magnetization of the magnetic recording layer.
Similar to the thermally assisted magnetic recording method, the microwave assisted magnetic recording method also can use a material having high magnetic anisotropy constant Ku for a magnetic layer of the magnetic recording medium. Therefore, size reduction of magnetic particles of the magnetic recording medium can be achieved while maintaining thermal stability.
In using the above-described magnetic storage apparatus that uses the thermally assisted magnetic recording method or the microwave assisted magnetic recording method, there is a demand for further reducing the size of magnetic crystal particles along with further sufficiently reducing coupling exchange between magnetic crystal particles, so that a medium having high SN ratio can be obtained. In order to satisfy this demand, it is effective to add SiO2 or carbon (C) to the magnetic layer as a grain boundary phase material as described above.
However, if a large amount of grain boundary phase material is added for obtaining a medium having high SN ratio in a case of using the magnetic storage apparatus, a problem of reduction of magnetic anisotropy constant Ku due to degradation of the degree of order of magnetic layer crystal particles (crystal particles of alloy that are included in a magnetic layer and have an L10 structure) such as FePt alloy crystal particles.